For many years steel sucker rods have been used in beam-pumped wells, although such rods have certain disadvantages such as low corrosion resistance and excessive aggregate weight to lift, especially in deep wells.
Recently, fiberglass sucker rods have been tested and found to be very satisfactory. Such rods are highly corrosive resistant, have substantially higher tensile strength than steel on a strength-to-weight basis, and weigh approximately one-third as much as a steel rod of substantially the same diameter.
In order to connect the fiberglass rods end-to-end, tubular metal end fittings have been required for encasing and gripping the end portions of the rods. The inside of the fitting is sometimes provided with a longitudinal series of inclined annular grooves each concentric to the tube axis forming wedge-shaped annular voids between the inner surface of the fitting and the outer cylindrical surface of the rod. The voids are filled with a potting compound such as an epoxy resin which bonds to the rod but does not adhere to the inner surfaces of the metal tube, so that the slip of the resin wedges on the tube surface due to tensile forces on the rod produces radial compressive holding forces to grip the rod.
The main disadvantage of the foregoing concentric wedge construction is that the radial compression forces generated by each resin wedge are applied to the rod at the small end of the wedge in a single plane at right angles to its longitudinal axis. This results in displacing the outer glass fibers inwardly and placing them in a preloaded tensile condition over the underlying greater amount of internal fibers, and the resulting stacking of the inner fibers resists radial displacement thereof and aggravates the stress differential between the inner and outer fibers. Thus, the combination of compression and displacement due to high tensile loads tends to overstress the outer fibers and cause failure thereof.